As is known in the art, simultaneous Transmit and Receive (STAR) is an emerging area of interest that offers significant benefits for many applications including communications, radar, spectral sensing. and multifunctional systems. One challenge in implementing practical STAR is mitigating the effects of strong in-band self-interference (SI) at the receiver(s). Various techniques have been proposed to accomplish this which may be broadly categorized into several domains.
Propagation domain methods include natural antenna isolation from physical separation and placement, along with polarization diversity and decoupling structures. Temporal domain methods involve subtracting a filtered copy of the transmitted waveform from the received waveform and may be performed in the analog domain, or in the digital domain using adaptive filtering and non-linear equalization. Finally, spatial domain methods use multiple transmit (receive) antennas to create spatial nulls at one or more receive (transmit) antennae.
To effectively enable STAR, multiple techniques are often combined, typically with an analog circuit layer to reduce transmit noise and to avoid receiver saturation followed by a digital cancellation circuit layer. For large arrays, many single-channel analog domain techniques are impractical due to dense element spacing and the multitude of transmit and receive channel pairs.
Spatial domain cancellation is, of course, a natural choice for phased array antennas. High-isolation apertures have been proposed to achieve spatial domain cancellation using symmetrically placed antenna elements, however these systems are not suitable for general far-field directional beamforming. Another solution for phased array antennas has been to combine spatial beamforming and digital cancellation, which has been previously explored for the related problem of full-duplex multiple-input multiple-output (MIMO) radios. The overall performance of existing techniques, however, is ultimately limited by the dynamic range of the individual transmit and receive channels. This represents a challenge for high power array antennas which require extremely high isolation, often of 140 dB or more, to support STAR, but whose transmitters may operate near saturation and produce significant distortion and noise.